Arduino code to wake up upon interrupt from a bluetooth module and toggle a pin each time (here controlling a relay hence relayPin).

btWake.c

#include <avr/sleep.h>
#include <SoftwareSerial.h>

SoftwareSerial blueSerial(3, 9);           // RX, TX pins
int relayActive = false;
int relayPin = 7;

void setup() {
  blueSerial.begin(9600);                  // Set baud rate
  pinMode(relayPin, OUTPUT);
  digitalWrite(relayPin, HIGH);
  attachInterrupt(1, toggleRelay, RISING); // Interupt for wake up
  
  blueSerial.print("Initialised\n\r");
  
  set_sleep_mode(SLEEP_MODE_PWR_DOWN);     // Lowest power mode
  sleep_enable();
  sleep_mode();                            // Go to sleep
}

void loop() {
  sleep_mode();
}

void toggleRelay() {
  blueSerial.print("Woah much intteruption!\n\r");
  if (!relayActive) {
    digitalWrite(relayPin, LOW);
  } else {
    digitalWrite(relayPin, HIGH);
  }
  relayActive = !relayActive;
}

It's clear now thank you. Is it appropriate to use the hc06 bluetooth module according to this code? Also do I need to use tx-rx pins or digital 1 / 2 (interrupt) pins?

No worries, I had this running with either a HC05 or HC06 module, so that should be fine. I think you'll need tx-rx pins for the serial then the mapping to a digital pin to attach the interrupt to. If software serial already does that then you should be set.

So should this command be used:

"attachInterrupt(digitalPinToInterrupt(RX_PIN)"

instead of ;

"attachInterrupt(1,

also even though the bluetooth connection is established, ("Initialised\n\r") message is not coming to the application screen I use on the phone.